Stockholders' equity (Shares Repurchase Program) (Details) - USD ($) $ / shares in Units, shares in Millions		3 Months Ended												12 Months Ended
	Mar. 06, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 05, 2018	Jan. 31, 2018
Equity [Abstract]
Total stock repurchases, Shares		4.5	4.4	6.2	3.4	6.7	4.4	3.9	4.7	1.2	4.6	3.3	2.9	18.5	19.7	12.0
Total stock repurchases		$ 796,000,000	$ 769,000,000	$ 1,006,000,000	$ 555,000,000	$ 999,000,000	$ 747,000,000	$ 591,000,000	$ 690,000,000	$ 184,000,000	$ 703,000,000	$ 515,000,000	$ 451,000,000	$ 3,126,000,000	$ 3,027,000,000	$ 1,853,000,000
Amount available for stock repurchases under a board approved stock repurchase plan		$ 4,400,000,000												$ 4,400,000,000
Subsequent event [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, additional authorized amount																		$ 10,000,000,000.0
Stock repurchase program, tender offer amount																	$ 10,000,000,000
Forecast [Member] | Maximum [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, cost per share	$ 200
Forecast [Member] | Minimum [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, cost per share	$ 175